Noob trying to connect Macbook to old speakers -- External soundcard, DAC, or media streamer?

For <150 bucks, we are not talking top flight DAC.

Consider just getting an Airport Express, which will let you move your macbook away from the receiver.  This can be upgraded to use a dedicated DAC later, via the optical out.

A better DAC would be to use a music streamer, which hooks up via USB

http://www.google.com/products/catalog?client=safari&rls=en&q=music+streamer&oe=UTF-8&um=1&ie=UTF-8&tbm=shop&cid=11566938663836358933&sa=X&ei=WyBDTtLqJqjw0gG_zsXtCQ&ved=0CCsQ8wIwAQ#ps-sellers

This is right at your budget.

To answer your original question, a DAC takes a digital input and produces an analog output. The Macbook's digital output should be fine. In fact even the analog might be, but the digital definitely is.

Your receiver doesn't have digital inputs, so you need to convert digital to analog. The two most common choices are:

* Take a digital output from the Macbook and use a DAC that takes the digital output from the Macbook (which most will). That output will be from a laser in the Macbook, so it's light. You typically use an optical cable that has a Macbook connector on one end and whatever the DAC wants on the other (probably Toslink, which is the standard connector for optical audio). Connect the analog output of the DAC to your receiver.

* Get a separate DAC that connects to the PC via USB. Again, connect the analog output to your receiver. 

A DAC with USB input may also be referred to as a "sound card", although I find it odd, since it isn't really a sound card. Both alternatives above are really the same, except one uses optical input and the other USB input.

I don't know the products available at $150, so I can't tell you whether a DAC with USB input or with optical input at that price range will be better. In theory optical is a slightly cleaner way to feed the signal than USB, but at this price level it's all done with single chips, and there may not be a real difference in quality.